Hyperthyroidism Flashcards
Give a brief overview of how T3 and T4 synthesis works
TSH travels from anterior pituitary In blood to follicular cells in the thyroid
This allows for the uptake of iodide
Iodide undergoes iodination your form iodine
Thrroglobulin is released into the colloid
Thyroid peroxidase enzyme is also released which is important for iodination
Iodine and thyroglobulin undergo iodination to form MIT and DIT
These undergo coupling reactions to form T3 and T4
What is the hypothalmo-pituitary-thyroid axis and what is the result?
TRH (thyrotrophin releasing hormone) from hypothalamus ->
TSH (thyroid stimulating hormone) from anterior pituitary ->
T3 and T4 from thyroid
T3 and 4 have an inhibitory effect on both TRH and TSH
The role of T4 (which is deiodinated to the active form T3) and T3 is to increase basal metabolic rate
How is primary hypothyroidism treated?
Primary means thyroid is damaged so can’t make T4
Give levothyroxine
Adjust dose (increase) until TSH levels go down to normal range (in primary hypothyroidism TSH will be high)
What is graves disease?
Causes hyperthyroidism
Autoimmune
Anti TSH-receptor antibodies bind to TSH receptors in the thyroid which causes excess T4 to be made
Causes a smooth goitre
What are the symptoms of Graves’ disease?
Perspiration
Exophthalmos
Pre tibial myxoedema
Tremor
Goitre
Weight loss
Increased appetite
Tachycardia
Palpitations
Diarrhoea
Heat intolerance
All are due to increased basal metabolic rate
How does Graves’ disease cause exophthalmos?
Graves is auto immune
Other antibodies bind to muscles behind the eye, causing the muscles to grow, pushing the eye out
This can threaten visionary
What is Pretibial myxoedema and what causes it in graves?
Swelling of the lower extremities (before the tibia). Doesn’t involve fluid (unlike pitting oedema which occurs in heart failure), it is an increase in soft tissue
It is caused by a different antibody that those that cause high T4 and exophthalmos
(Don’t confuse it with myxoedema that is advanced hypothyroidism)
What does a radioiodine scan look like in Graves’ disease?
Diffuse enlargement and engorgement of thyroid gland, also symmetrical
The whole gland is overactive so takes up the radioiodine equally so you can see the whole shape of the gland, it will also be enlarged.
The pyramidal lobe often lights up too
What does a radioiodine scan look like with a hyper functioning adenoma?
Adenoma will be a large lump on one side of the gland usually. The goitre will be asymmetric.
The rest of the gland shrinks
It isn’t as active so doesn’t take up as much iodine and doesn’t appear as bright as the thyroid in Graves
The lump will show up the most as this takes up more iodine
What are the effects of iodithyronines on the sympathetic nervous system?
Sensitises beta 2 adrinergic receptors to ambient levels of adrenaline and noradrenaline
:. There is apparent sympathetic activation
Leads to: Tachycardia Palpitations Hand tremor Lid lag (eyelids held open)
What are the symptoms of hyperthyroidism (too much thyroxine) ?
Weight loss despite increased appetite
Breathlessness
Tachycardia
Palpitations
Sweating
Heat intolerance (hot in winter)
Diarrhoea
Lid lag (and other sympathetic features)
What is a thyroid storm?
Happens when people, for whatever reason don’t want to go to hospital
It has a 50% mortality if untreated
Blood results confirm hyperthyroidism
Needs aggressive treatment
Symptoms: Hyperpyrexia >41°c Accelerated tachycardia (>170) /arrhythmia Cardiac failure Delirium/frank psychosis Hepaticellar dysfunction; jaundice
Must have 2 or more of these symptoms
How is hyperthyroidism treated?
Three options:
Surgery (thyroidectomy)
Radioiodine
Drugs
What are the types of drugs used in hyperthyroidism treatment?
- Thionamides (anti thyroid drugs):
Propylthiouracil (PTU)
Carbimazole(CBZ)
Used as daily treatment for Graves and toxic thyroid nodule - Potassium iodide:
Large loads of iodine can completely stop the thyroid gland working for a while - Radioiodine:
Can kill the gland - Beta blockers:
These are given along with thionamides as they stop sympathetic activity so help with some of the symptoms
Propanolol used as it is non specific (unlike bisoprolol which only treats heart sympathetic symptoms)
What is the mechanism of action of thionamides? And how long are they prescribed?
They inhibit the action of thyroid peroxidase enzymes (help with iodination). This stops T4 synthesis
The biochemical effect takes hours
But the clinical effect takes weeks. This is because there are stores of leftover thyroxine which have to be used up
Therefore it is often given alongside propanolol (beta blocker) which rapidly reduces tremor and tachycardia
Usually aim to stop anti thyroid drug treatment after 18 months
Patient is then reviewed periodically to test for relapse/remission
Only half of patients are cured
What are some side effects of thionamides?
Agranulocytosis (reduction in neutrophils) - rare and reversible on withdrawal of drug
Rashes - more common
Why are beta blockers given along with thionamides?
Takes several weeks for thionamides to have an effect
So beta blockers are given in the mean time to reduce symptoms caused by increased sympathetic activity (tachycardia, tremor)
A non selective beta blocker is used
Eg. Propanolol
As selective ones eg. Bisoprolol would only treat the heart sympathetic symptoms
When is potassium iodide used? And what is it’s mechanism of action?
Given in doses at least 30x average daily requirement
Used to prepeare patients for surgery
And in severe thyrotoxic crisis (thyroid storm)
Causes inhibition of thyroid hormone synthesis and secretion. By the Wolff-Chaikoff effect
Inhibits iodination of thyroglobulin, and inhibits Peroxide generation and thyroperoxidases
Reduced hyperthyroid symptoms in 1-2 days
Vascularity and size of glad reduce in 10-14 days
Can’t use long term as eventually the thyroid ends up actually using all of that iodine
What are the risks of thyroid surgery?
Risk of voice change due to proximity to the recurrent laryngeal nerve
Also risk to parathyroid glands - role in calcium regulation, it increases calcium levels
You’ll get a scar
And anaesthetic risks
How does radioiodine therapy work?
Swallow capsule containing 370 MBq of the isotope I-131
It is contradicted in pregnancy as radioactivity will harm the baby
The person will be radioactive so will need to avoid children and pregnant people for a few days
Iodine is expensive, so for scans 99-Tc Pertechnetate is used
What is viral thyroiditis and it’s symptoms?
Hyperthyroidism caused by a viral infection
Aka de quervains thyroiditis
Hyper at first, as stores are released, then hypo later as virus is replicating so thyroid isn’t making hormones Symptoms: Painful dysphagia Hyperthyroiditis Pyrexia Thyroid inflammation (usually more on one side) Pain Tender paratracheal lymph nodes
What is the history/process of viral thyroiditis?
Virus attacks the thyroid gland causing pain and tenderness
Thyroid stops making thyroxine and makes virus instead
Therefore there is no iodine uptake at all
During the first 4 months stored T4 is released
After this stores run out so you become hypothyroid for a month
Then resolution occurs
Do not treat it, you’ll just mess up the timeline
After 3 months there is a slow recovery
(Post partum thyroiditis is similar but there is no pain and you have a baby). Caused because pregnancy modulates the immune system
What does a viral thyroiditis radioiodine scan look like?
Nothing at all
Coz there is Zero iodine uptake as the thyroid is busy making virus
